Phosphodiesterases (PDEs) are a superfamily of enzymes encoded by 21 genes and subdivided into 11 distinct families according to structural and functional properties. These enzymes metabolically inactivate the ubiquitous intracellular second messengers, cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP); PDEs selectively catalyze the hydrolysis of the 3′-ester bond, forming the inactive 5′-monophosphate. On the basis of substrate specificity, the PDE families can be further classified into three groups: i) the cAMP-PDEs (PDE4, PDE7, PDE8), ii) the cGMP-PDEs (PDE5, PDE6 and PDE9), and iii) the dual-substrate PDEs (PDE1, PDE2, PDE3, PDE10 and PDE11).
The cAMP and cGMP are involved in the regulation of virtually every physiological process such as pro-inflammatory mediator production and action, ion channel function, muscle relaxation, learning and memory formation, differentiation, apoptosis, lipogenesis, glycogenolysis and gluconeogenesis. Especially, in neurons, these second messengers have important role in the regulation of synaptic transmission as well as in neuronal differentiation and survival (Nat. Rev. Drug Discov. 2006, vol. 5: 660-670). Regulation of these processes by cAMP and cGMP are accompanied by activation of protein kinase A (PKA) and protein kinase G (PKG), which in turn phosphorylate a variety of substrates, including transcription factors, ion channels and receptors that regulate a variety of physiological processes. Intracellular cAMP and cGMP concentrations seem to be temporally, spatially, and functionally compartmentalized by regulation of adenyl and guanyl cyclases in response to extracellular signaling and their degradation by PDEs (Circ. Res. 2007, vol. 100(7): 950-966). PDEs provide the only means of degrading the cyclic nucleotides cAMP and cGMP in cells, thus PDEs play an essential role in cyclic nucleotide signaling. Thereby, PDEs could be promising targets for various therapeutic drugs.
Phosphodiesterase 10A (PDE10A) was discovered in 1999 by three independent groups (Proc. Natl. Acad. Sci. USA 1999, vol. 96: 8991-8996, J. Biol. Chem. 1999, vol. 274: 18438-18445, Gene 1999, vol. 234: 109-117). Expression studies have shown that PDE10A has the most restricted distribution within the all known PDE families; the PDE10A mRNA is highly expressed only in brain and testes (Eur. J. Biochem. 1999, vol. 266: 1118-1127, J. Biol. Chem. 1999, vol. 274: 18438-18445). In the brain, mRNA and protein of PDE10A are highly enriched in medium spiny neurons (MSNs) of the striatum (Eur. J. Biochem. 1999, vol. 266: 1118-1127, Brain Res. 2003, vol. 985: 113-126). MSNs are classified into two groups: the MSN that express D1 dopamine receptors responsible for a direct (striatonigral) pathway and the MSN that express D2 dopamine receptors responsible for an indirect (striatopallidal) pathway. The function of direct pathway is to plan and execution, while indirect pathway is to act as a brake on behavioral activation. As PDE10A expresses in both MSNs, PDE10A inhibitors could activate both of these pathways. The antipsychotic efficacy of current medications, D2 or D2/5-HT2A antagonists, mainly derives from their activation of the indirect pathway in the striatum. As PDE10A inhibitors are able to activate this pathway, this suggests that PDE10A inhibitors are promising as antipsychotic drugs. The excessive D2 receptor antagonism in the brain by D2 antagonists causes problems of extrapyramidal side effects and hyperprolactinaemia. However the expression of PDE10A is limited to these striatal pathways in the brain, thus side effects by PDE10A inhibitors were expected to be weaker compared with current D2 antagonists. Regarding hyperprolactinaemia, PDE10A inhibitors would produce no prolactin elevation due to lack of D2 receptor antagonism in the pituitary. Moreover, the presence of PDE10A in a direct pathway makes it likely that PDE10A inhibition will have some advantage over current D2 antagonists; the direct pathway is thought to promote desired action, and activation of this pathway by PDE10A inhibitors may counteract extrapyramidal symptoms induced by excessive D2 receptor antagonism. In addition, activation of this pathway could facilitate striatal-thalamic outflow, promoting the execution of procedural strategies. Furthermore, enhancement of second messenger levels without blockade of dopamine and/or other neurotransmitter receptors may also provide therapeutic advantages with fewer adverse side-effects (e.g., hyperprolactinaemia and weight gain) compared with current antipsychotics. This unique distribution and function in the brain indicates that PDE10A represents an important new target for the treatment of neurological and psychiatric disorders, in particular psychotic disorders like schizophrenia.
Patent document 1 discloses, as PDE10A inhibitors, a compound of the following formula:
and the following compounds

Patent document 2 discloses, as PDE10A inhibitors, a compound of the following formula:
and the following compounds:

Patent document 3 discloses, as PDE10A inhibitors, a compound of the following formula:
and the following compounds:

Patent document 4 discloses, as a PDE10A inhibitor, a compound of the following formula:
wherein Z is

Patent document 5 discloses, as a PDE10A inhibitor, a compound of the following formula:

Patent document 6 discloses, as PDE10A inhibitors, a compound of the following formula:
and the following compounds:

Patent document 7 discloses a compound of the following formula:
and the following compounds

Patent document 8 discloses the following compounds:

Patent document 9 discloses the following compound:

Patent document 10 discloses the following compound:

Patent document 11 discloses, as a PDE10A inhibitor, the following compound:
